Porous stretched article of polypropylene-based resin and process for its preparation

Abstract: Disclosed are a process for producing a porous stretched article of β-crystalline polypropylene-based resin, the process comprising the steps of crystallizing a melt of a polypropylene-based resin composition at a temperature of 15 to 140°C to form a solidified product containing β-form crystals and stretching the resulting solidified product at a temprature which is higher than 20 °C but lower than the melting point of the β-form crystals in said solidified product, the polypropylene-based resin composition comprising a polypropylene-based resin and 0.0001 to 5 parts by weight, per 100 parts by weight of the polypropylene-based resin, of at least one specific amide compound; and a porous stretched article produced by the process.

Description: The present invention relates to a process for producing a porous stretched product of β-crystalline polypropylene-based resin and to the stretched product of polypropylene-based resin produced by this process. Various processes for preparing porous polypropylene-based materials are known. For example, the following processes are known. (1) a process comprising admixing an inorganic substance and/or a non-compatible resin with polypropylene, extruding the mixture into a film and orienting or drawing the film to thereby obtain a porous film (JP-A-50-116561 (1975), etc.); and (2) a process comprising admixing an inorganic substance or an organic liquid with polypropylene, extracting the mixture with a solvent or alkali, extruding the mixture into a film and stretching the film to thereby obtain a porous film (JP-A-58-59072 (1983),etc.). The process (1) has the drawbacks that the inorganic substance bleeds out, and that the resulting film has a reduced elongation at break, and the process (2) entails the drawback of requiring troublesome procedures such as time-consuming extraction step. Also known is a process (3) for producing a porous film, utilizing a crystal form (β-form crystal) peculiar to polypropylene (JP-B-55-40411 (1980), etc.). However, this process has difficulty in forming a sufficient amount of β-form crystals under practical conditions and does not give a porous film having satisfactory porosity. Processes heretofore proposed for producing β-form crystals of polypropylene include, for example, a process in which a melt of polypropylene is crystallized under a temperature gradient, or a process using a small amount of a β-nucleating agent (γ-quinacridone) (POLYMER LETTERS, 6, 539-546, (1968)). The former process has the shortcomings of taking a long time and giving only a small quantity of product. The latter process is deficient in requiring a specially designed apparatus and procedure for admixing the nucleating agent with polypropylene, and has a further problem of being low in usefulness in that it is necessary to conduct the crystallization step at a high temperature in order to form a certain increased amount of β-form crystals in the unoriented product, thereby requiring a long time for molding. In addition, the process has additional problem that the product is red colored. The present inventors have proposed various novel nucleating agents for increasing the content of β-form crystals to obviate the foregoing problems (U.S. Application Ser. No.08/003,659). The object of the present invention is to overcome the drawbacks of conventional technologies, to improve the above process (3) and to thereby provide a novel and useful process for preparing under practical conditions a porous polypropylene-based material exhibiting satisfactory properties. This object has been achieved by the surprising finding that the contemplated stretched porous article can be obtained by crystallizing at a specific temperature a melt of a polypropylene containing an amide compound of specific structure as a β-nucleating agent in a predetermined amount, and stretching the resulting product at a specific temperature. Thus, the present invention provides a process for producing a porous stretched article of a β-crystalline polypropylene-based resin wherein the stretched article has a porosity of at least 10%, the process comprising the steps of crystallizing a melt of a polypropylene-based resin composition at a temperature of 15 to 140°C to form a solidified product containing β-form crystals and stretching the resulting solidified product at a temperature which is higher than 20 °C but lower than the melting point of the β-form crystals in said solidified product, the polypropylene-based resin composition comprising a polypropylene-based resin and 0.0001 to 5 parts by weight, per 100 parts by weight of the polypropylene-based resin, of at least one amide compound of the formula R 2 -NHCO-R 1 -CONH-R 3 wherein R 1 is a C 1-24 saturated or unsaturated aliphatic dicarboxylic acid residue, a C 4-28 saturated or unsaturated alicycic dicarboxylic acid residue or a C 6-28 aromatic dicarboxylic acid residue; R 2 and R 3 are the same or different and each represents a C 3-18 cycloalkyl group, a C 3-12 cycloalkenyl group, or a group of the formula wherein R 4 is a hydrogen atom, C 1-12 straight- or branched-chain alkyl group, a C 2-12 straight- or branched-chain alkenyl group, a C 6-10 cycloalkyl group or phenyl group; R 5 is a C 1-12 straight- or branched-chain alkyl group or a C 2-12 straight- or branched-chain alkenyl group; and R 6 and R 7 each represents a C 1-4 straight- or branched-chain alkylene group. The polypropylene-based resin for use in the present invention is a polymer composed predominantly of polypropylene. Examples of such polymers are propylene homopolymers, copolymers of propylene as a predominant comonomer and at least one comonomer selected from the group consisting of ethylene and at least one 1-alkene (including random copolymers and block copolymers), propylene-ethylene copolymers which further contain other comonomer(s) (such as those which further contain 5-ethylidene-2-norbornene, 5--methylene-2-norbornene or 1,4-hexadiene), propylene-styrene copolymers, etc. Also usable is a blend of polypropylene and a small quantity of a thermoplastic resin such as a high-density polyethylene, polybutene-1 or poly-4-methylpentene-1. The melt flow rate (hereinafter referred to briefly as "MFR"; measured in accordance with JIS K 6758-1981) of the polypropylene-based resin can be appropriately selected according to the molding method to be employed and is generally about 0.1 to 100 g/10 min and preferably about 0.5 to 50 g/10 min, from the standpoint of processability and mechanical properties and other properties of the resulting stretched article. In the present invention, amide compounds of the foregoing formula (1) are used. Amide compounds of the formula (1) The amide compounds of the formula (1) are known or can be easily prepared by conventional methods, for example, by subjecting an aliphatic, alicyclic or aromatic dicarboxylic acid of the the formula HOOC-R 1 -COOH wherein R 1 is as defined hereinbefore to amidation reaction with an alicyclic or aromatic monoamine of the formula R 2 -NH 2 or R 3 -NH 2 wherein R 2 and R 3 are as defined hereinbefore. As the dicarboxylic acid of the formula (1a), there may be mentioned the aliphatic, alicyclic or aromatic dicarboxylic acid corresponding to R 1 . Thus, R 1 is a residue formed by elimination of the two carboxyl groups of one of the following aliphatic, alicylic and aromatic dicarboxylic acids. The aliphatic dicarboxylic acid specifically includes C 3-26 , preferably C 3-14 saturated or unsaturated aliphatic dicarboxylic acids, such as malonic acid, diphenylmalonic acid, succinic acid, phenylsuccinic acid, diphenylsuccinic acid, glutaric acid, 3,3-dimethylglutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, 1,12-dodecanedioic acid, 1,14-tetradecanedioic acid, 1,18-octadecanedioic acid and so on. The alicyclic dicarboxylic acid specifically includes C 6-30 , preferably C 8-12 saturated or unsaturated alicyclic dicarboxylic acids such as 1,2-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,4-cyclohexanediacetic acid and so on. The aromatic dicarboxylic acid specifically includes C 8-30 , preferably C 8-22 aromatic dicarboxylic acids such as p-phenylenediacetic acid, p-phenylenediethanoic acid, phthalic acid, 4-tert-butylphthalic acid, isophthalic acid, 5-tert-butylisophthalic acid, terephthalic acid, 1,8-naphthalic acid, 1,4-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenic acid, 3,3'-biphenyldicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 4,4'-binaphthyldicarboxylic acid, bis(3-carboxyphenyl)methane, bis(4-carboxyphenyl)methane, 2,2-bis(3-carboxyphenyl)propane, 2,2-bis(4-carboxyphenyl)propane, 3,3'-sulfonyldibenzoic acid, 4,4'-sulfonyldibenzoic acid, 3,3'-oxydibenzoic acid, 4,4'-oxydibenzoic acid, 3,3'-carbonyldibenzoic acid, 4,4'-carbonyldibenzoic acid, 3,3'-thiodibenzoic acid, 4,4'-thiodibenzoic acid, 4,4'-(p-phenylenedioxy)dibenzoic acid, 4,4'-isophthaloyldibenzoic acid, 4,4'-terephthaloyldibenzoic acid, dithiosalicylic acid and so on. On the other hand, the monoamine to be used is the alicyclic or aromatic monoamine corresponding to R 2 or R 3 , i.e., R 2 -NH 2 or R 3 -NH 2 . The alicyclic monoamine particularly includes C 3 -C 18 cycloalkylamines, C 3 -C 12 cycloalkenylamines, (wherein R 5 is as defined above), (wherein R 7 is as defined above) and the like, and specifically includes, among others, cyclopropylamine, cyclobutylamine, cyclopentylamine, cyclohexylamine, 2-methylcyclohexylamine, 3-methylcyclohexylamine, 4-methylcyclohexylamine, 2-ethylcyclohexylamine, 4-ethylcyclohexylamine, 2-propylcyclohexylamine, 2-isopropylcyclohexylamine, 4-propylcyclohexylamine, 4-isopropylcyclohexylamine, 2-tert-butylcyclohexylamine, 4-n-butylcyclohexylamine, 4-isobutylcyclohexylamine, 4-sec-butylcyclohexylamine, 4-tert-butylcyclohexylamine, 4-n-amylcyclohexylamine, 4-isoamylcyclohexylamine, 4-sec-amylcyclohexylamine, 4-tert-amylcyclohexylamine, 4-hexylcyclohexylamine, 4-heptylcyclohexylamine, 4-octylcyclohexylamine, 4-nonylcyclohexylamine, 4-decylcyclohexylamine, 4-undecylcyclohexylamine, 4-dodecylcyclohexylamine, 4-cyclohexylcyclohexylamine, 4-phenylcyclohexylamine, cycloheptylamine, cyclododecylamine, cyclohexylmethylamine, α-cyclohexylethylamine, β-cyclohexylethylamine, α-cyclohexylpropylamine, β-cyclohexylpropylamine, γ-cyclohexylpropylamine and so on. The aromatic monoamine particularly includes (wherein R 4 is as defined above), (wherein R 6 is as defined above) and the like, and specifically includes, among others, aniline, o-toluidine, m-toluidine, p-toluidine, o-ethylaniline, p-ethylaniline, o-propylaniline, m-propylaniline, p-propylaniline, o-cumidine, m-cumidine, p-cumidine, o-tert-butylaniline, p-n-butylaniline, p-isobutylaniline, p-sec-butylaniline, p-tert-butylaniline, p-n-amylaniline, p-isoamylaniline, p-sec-amylaniline, p-tert-amylaniline, p-hexylaniline, p-heptylaniline, p-octylaniline, p-nonylaniline, p-decylaniline, p-undecylaniline, p-dodecylaniline, p-cyclohexylaniline, o-aminodiphenyl, m-aminodiphenyl, p-aminodiphenyl, p-aminostyrene, benzylamine, α-phenylethylamine, β-phenylethylamine, α-phenylpropylamine, β-phenylpropylamine and γ-phenylpropylamine. Among the amide compounds of the formula (1), those which are particularly preferable include N,N'-diphenylhexanediamide, N,N'-dicyclohexylterephthalamide, or N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide. Said amide compounds of the formula (1) may be added during the preparation of the polypropylene-based resin or may be mixed with a resin prepared separately. The amount of the amide compound to be added is 0.0001 to 5 parts by weight, preferably 0.001 to 1 part by weight, per 100 parts by weight of the polypropylene-based resin. If the proportion of the amide compound is less than 0.0001 part by weight, the formation of β-form crystals may not be sufficient, while the use of the amide compound in excess of 5 parts by weight may not be rewarded with a commensurate effect and be uneconomical, and tends to reduce the elongation at break and tensile strength at break of the stretched article. The temperature at which the polypropylene-based resin composition is melted is in the range of about 190 to 300°C, preferably about 200 to 280 °C. The crystallizing temperature (cooling temperature) of the melt of the resin composition in the process of this invention is in the range of about 15 to 140°C, preferably about 20 to 120°C, more preferably about 70 to 120°C. The content of β-form crystals (K value) in the unstretched product thus obtained is about 0.5 to 1.0 which is a range suitable for practical use. This crystallizing step is performed, for example, by extruding the polypropylene-based resin containing at least one of the β-nucleating agents of the formula (1) into a sheet, and then cooling and solidifying the thus obtained sheet by a chill roll maintained at a temperature within the above-specified temperature range to thereby form β-form crystals, as will be described in examples. The obtained unstretched product of polypropylene-based resin containing β-form crystals is then stretched in a conventional manner but under a specific temperature condition. It is important in the present invention to stretch the unstretched product of polypropylene-based resin containing β-form crystals at a temperature which is higher than 20 °C but lower than the melting point of the β-form crystals in said unstretched product. The melting point of the β-form crystals in said unstretched product can be easily measured by differential scanning calorimetry, and is up to about 150 °C and depends on the kind of polypropylene-based resin. For achieving higher porosity, the upper limit of the temperature at which the unstretched product is stretched is preferably a temperature which is lower than the melting point of the β-form crystals in said unstretched product by at least 10 degrees. In other words, it is preferable to conduct stretching at a temperature which is lower than Ts (°C) defined by the following equation Ts = Tm - 10 wherein Tm is the melting point (°C) of the β-form crystals in the unstretched product. From the standpoint of the stretching operation, the stretching temperature is preferably not lower than 70 °C, more preferably not lower than 90 °C. The stretched product of the present invention having a porosity of at least 10%, preferably about 20 to 80 % an be prepared only by conducting the crystallization step at a temperature within the above temperature range, and then conducting stretching at a temperature within the above-specified temperature range. Stretching mode to be employed is uniaxial stretching or simultaneous or successive biaxial stretching. The uniaxial stretching and the simultaneous or successive biaxial stretching can be conducted by conventional methods. In case of biaxial stretching, the extent of stretching is not specifically limited, and a preferred draw ratio is, for example, about 1.2 to 5.0, preferably 1.7 to 2.8, in the machine direction or longitudinal direction and about 1.2 to 5.0, preferably 1.8 to 2.6, in the transverse direction. In case of uniaxial stretching, the extent of stretching is also not specifically limited, and a preferred draw ratio is, for example, about 1.5 to 6.0, preferably 2.5 to 3. The pressure conditions to be employed in the process are variable over a wide range and can be any of normal pressure, increased pressure and reduced pressure. If required, the polypropylene-based resin composition to be used in the present invention may contain a variety of conventional additives such as a stabilizer (e.g. epoxy compounds), an antioxidant (e.g. phenol compounds and phosphorous acid ester compounds), an ultraviolet absorber (e.g. benzophenone compounds and benzotriazole compounds), a neutralizer, a nucleating agent, an antistatic agent, an antiblocking agent, a lubricant (e.g. aliphatic hydrocarbons, higher fatty acids and the alkali metal salts or alkaline earth metal salts thereof, fatty acid esters, higher fatty acid amides, rosin derivatives, etc.), a colorant, an elastomer, and a mineral (e.g. talc and hydrotalcite), each within a range not interfering with the effect of the invention. The stretched or oriented product of polypropylene-based resin thus obtained is used to produce an opaque film which is excellent in whiteness, hiding power, printability and writability, or to produce a porous membrane having a multiplicity of interpenetrating pores. The opaque film finds wide applications as materials for printing sheets, tracing paper, food packaging films, etc. The porous membranes are extensively used, for example, in the form of films or hollow yarns, for various applications as membranes for moisture-permeating and water-proofing purposes, as filters or separating membranes for purification of air or disposal of waste water, as medical materials for plasma-separating membranes or artificial lungs, as battery separators useful for batteries and electrolysis, and so on. EXAMPLES The following examples and comparative examples are intended to describe the invention in further detail. In these examples and comparative examples, the content of β-form crystals and the porosity were determined by the following methods. (1) Determination of β-form crystal content (i) X-ray method A sample (polypropylene-based resin cast film, unstretched film) was subjected to X-ray diffraction and a K value was obtained by the folllowing equation. K value= H (β 1 ) H (β 1 ) + H (α 1 ) + H (α 2 ) + H (α 3 ) wherein: H (β 1 ) is a reflection intensity (height) on (300) plane of β-form crystal; H (α 1 ) is a reflection intensity (height) on (110) plane of α-form crystal; H (α 2 ) is a reflection intensity (height) on (040) plane of α-form crystal; and H (α 3 ) is a reflection intensity (height) on (130) plane of α-form crystal. (ii) DSC method A sample (about 5 mg), cut from the polypropylene-based resin cast film (unstretched film) obtained in each of Examples and Comparative Examples, is set in the sample holder of a differential scanning calorimeter (DSC) and heated in a nitrogen gas atmosphere at a rate of 20°C/min. From the peak areas of the α- and β-form crystals on the DSC thermogram thus obtained, the β-form crystal content (area %, hereinafter "DSC %") is calculated. (2) Determination of porosity Salad oil (ρ = 0.925 g/cm 3 ) was impregnated into a sample stretched film by immersing the sample in the salad oil at room temperature for 5 minutes. The salad oil adhered to the surface of the sample was wiped with a methanol-impregnated paper. Then the weight of the sample film was measured and the volume of the salad oil impregnated into the film was calculated. The porosity was given by the following equation α=(Vo/Vf) X 100 wherein α is the porosity (%), Vo is a volume of salad oil impregnated into the film; and Vf is an apparent volume of the sample film. Examples 1 to 7 To 100 parts by weight of a propylene homopolymer powder with MFR of 14 g/10 min (hereinafter called "PP (1)") was added 0.1 part by weight of N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide (hereinafter called "nucleating agent 1") as a (3-nucleating agent. The mixture was milled in a Henschel mixer and pelletized at 240°C. The resulting pellets were extruded by a T-die extruder into a sheet at a resin temperature of 240°C, and the sheet was cooled and solidified by a chill roll maintained at a temperature shown in Table 1 (Cooling temperature) to give an unstretched polypropylene cast film having a thickness of 200 µm. Table 1 below shows the content of β-form crystals (K value and DSC%) each of the unstretched films thus obtained. Then each of the unstretched films was uniaxially or biaxially stretched into a white opaque film under the conditions shown in Table 1. In each of Examples 1-7 (and also in each of Examples 8-17 and Comparative Example 1 that follow), the stretching temperature was lower than the melting point of the β-form crystals contained in the unstretched product as determined by a differential scanning calorimeter (DSC). The melting point of the β-form crystals of PP(1) is 152 °C. Then, salad oil was impregnated into these stretched films, and the films were thereby all rendered translucent. This means that they were porous films having excellent interpenetrating pore structure. Table 1 shows the thickness of the stretched film, and the porosity (%) of the stretched film as calculated from the amount of impregnated salad oil. Example 8 A stretched film was prepared in a manner similar to Example 3 using a propylene homopolymer powder with MFR of 4.3 g/10 min (hereinafter called "PP (2)"). The melting point of the β-form crystals of PP(2) is 147 °C. The obtained film was a porous film having excellent interpenetrating pore structure. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film and the thickness and porosity (%) of the stretched film. Example 9 A stretched film was prepared in a manner similar to Example 3 using a propylene homopolymer powder with MFR of 1.4 g/10 min (hereinafter called "PP (3)"). The melting point of the β-form crystals of PP(3) is 148 °C. The obtained film was a porous film having excellent interpenetrating pore structure. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film and the thickness and porosity of the stretched film. Example 10 Except that PP (3) was used, the procedure of Example 6 was substantially followed to provide an unstretched film. A stretched film was obtained by uniaxially drawing the unstretched film at 90°C. The stretched film was a porous film having excellent interpenetrating pore structure. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film and the thickness and porosity of the stretched film. Examples 11 and 12 Stretched films were prepared in a manner similar to Example 1 using different amounts of nucleating agent 1. The stretched films were porous films having excellent interpenetrating pore structure. Table 1 shows the content of β-form crystals (K value and DSC%) each of the unstretched films and the thickness and porosity each of the stretched films. Example 13 A stretched film was prepared in a manner similar to Example 1 using N,N'-dicyclohexylterephthalamide (hereinafter called "nucleating agent 2") as the β-nucleating agent. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film and the thickness and porosity of the stretched film. Example 14 A stretched film was prepared in a manner similar to Example 1 using N,N'-diphenylhexanediamide (hereinafter called "nucleating agent 3") as the β-nucleating agent. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film and the thickness and porosity of the stretched film. Comparative Example 1 Except that cooling and solidification were performed by a chill roll maintained at 10°C, the procedure of Example 1 was followed to provide an unstretched film. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film. A stretched film was obtained by uniaxially drawing the unstretched film at 130°C. The obtained film was translucent and had a low porosity, as shown in Table 1. Comparative Example 2 Except that cooling and solidification were performed by a chill roll maintained at 20°C, the procedure of Example 1 was followed to provide an unstretched film. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film. A stretched film was obtained by uniaxially drawing the unstretched film at 155°C. The obtained film was substantially transparent and no salad oil was impregnated into the film. Comparative Example 3 Except that a nucleating agent was not used, the procedure of Example 1 was otherwise repeated to provide an unstretched film. Table 1 shows the content of β-form crystals (K value and DSC%) of the unstretched film. A stretched film was obtained by biaxially stretching the unstretched film at 125°C. The obtained film was substantially transparent and no salad oil was impregnated into the film. When the amide compound of the present invention is used as a β-nucleating agent, a stretched porous product of a polypropylene-based resin can be easily prepared on a commercial basis under practical conditions. The stretched product is useful as materials for printing sheets, filtration membranes, separating membranes, etc.

Claims: A process for producing a porous stretched article of polypropylene-based resin wherein the stretched article has a porosity of at least 10%, the process comprising the steps of crystallizing a melt of a polypropylene-based resin composition at a temperature of 15 to 140°C to form a solidified product containing β-form crystals and stretching the resulting solidified product at a temperature which is higher than 20 °C but lower than the melting point of the β-form crystals in said solidified product, the polypropylene-based resin composition comprising a polypropylene-based resin and 0.0001 to 5 parts by weight, per 100 parts by weight of the polypropylene-based resin, of at least one amide compound of the formula R 2 -NHCO-R 1 -CONH-R 3 wherein R 1 is a C 1-24 saturated or unsaturated aliphatic dicarboxylic acid residue, a C 4-28 saturated or unsaturated alicyclic dicarboxylic acid residue or a C 6-28 aromatic dicarboxylic acid residue; R 2 and R 3 are the same or different and each represents a C 3-18 cycloalkyl group, a C 3-12 cycloalkenyl group, or a group of the formula wherein R 4 is a hydrogen atom, a C 1-12 straight- or branched-chain alkyl group, a C 2-12 straight- or branched-chain alkenyl group, a C 6-10 cycloalkyl group or phenyl group; R 5 is a C 1-12 straight- or branched-chain alkyl group or a C 2-12 straight- or branched-chain alkenyl group; and R 6 and R 7 each represents a C 1-4 straight- or branched-chain alkylene group. The process according to claim 1, wherein the solidified product is stretched at a temperature which is not lower than 70°C but lower than 150°C. A process according to claim 1, wherein the amide compound of the formula (1) is N,N'-diphenylhexanediamide, N,N'-dicyclohexylterephthalamide or N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide. A process according to any of claims 1 to 3, wherein said at least one amide compound is(are) used in an amount of 0.001 to 1 part by weight per 100 parts by weight of the polypropylene-based resin. A process according to any of claims 1 to 4, wherein the melt is crystallized at 20 to 120°C. A process according to claim 5, wherein the melt is crystallized at 70 to 120°C. A process according to claim 6, wherein the solidified product is stretched at a temperature which is not lower than 90 °C but lower than 150°C. A process according to any of claims 1 to 6, wherein the solidified product is stretched at a temperature which is not lower than 70 °C but lower than Ts (°C) which is defined by the equation Ts = Tm - 10 wherein Tm is the melting point (°C) of the β-form crystals in said product. A process according to claim 8, wherein the solidified product is stretched at a temperature which is not lower than 90 °C but lower than Ts (°C) which is defined by the equation Ts = Tm - 10 wherein Tm is the melting point (°C) of the β-form crystals in said product. A process according to any of claims 1 to 9, wherein the solidified product is stretched by uniaxial stretching. A process according to any of claims 1 to 9, wherein the solidified product is stretched by biaxial stretching. A process for producing a porous stretched article of polypropylene-based resin wherein the stretched article has a porosity of at least 10%, the process comprising the steps of crystallizing a melt of a polypropylene-based resin composition at a temperature of 15 to 140°C to form a solidified product containing β-form crystals and stretching the resulting solidified product at a temperature which is higher than 70 °C but lower than the melting point of the β-form crystals in said solidified product, the polypropylene-based resin composition comprising a polypropylene-based resin and 0.0001 to 5 parts by weight, per 100 parts by weight of the polypropylene-based resin, of an amide compound represented by the formula R 2 -NHCO-R 1 -CONH-R 3 wherein R 1 is a C 1-24 saturated or unsaturated aliphatic dicarboxylic acid residue, a C 4-28 saturated or unsaturated alicycic dicarboxylic acid residue or a C 6-28 aromatic dicarboxylic acid residue; R 2 and R 3 are the same or different and each represents a C 3-18 cycloalkyl group, a C 3-12 cycloalkenyl group, or a group of the formula wherein R 4 is a hydrogen atom, a C 1-12 straight- or branched-chain alkyl group, a C 2-12 straight- or branched-chain alkenyl group, a C 6-10 cycloalkyl group or phenyl group; R 5 is a C 1-12 straight- or branched-chain alkyl group or a C 2-12 straight- or branched-chain alkenyl group; and R 6 and R 7 are the same or different and each represents a C 1-4 straight- or branched-chain alkylene group. A porous stretched article of a polypropylene-based resin wherein the stretched article has a porosity of at least 10%, characterized in that it is obtainable by the process of any of claims 1 to 12.

Inventors: IKEDA NAOKI, KIMURA YOSHIHARU, MIZOGUCHI KAZUAKI, YOSHIMURA MASAFUMI, IKEDA, NAOKI, KIMURA, YOSHIHARU, MIZOGUCHI, KAZUAKI, YOSHIMURA, MASAFUMI
Applicants: NEW JAPAN CHEM CO LTD, NEW JAPAN CHEMICAL CO.,LTD.
Application year: 1994
Main classification: C08L23
IPC codes: B01D67, H01M2, B29C55, C08J5, C08K5, B01D71